The present invention relates in general to the design of light shutters as used in motion picture film projectors. More particularly, the present invention relates to improved shutter designs, which can be used in cinema projectors of standard design, as well as in projectors with improved Geneva Mechanisms and improved illumination systems.
Generally speaking, the state of the art motion picture film projector is little changed from those produced in the 1950""s, when the advent of robust color films and xenon arc lamps encouraged the manufacturer""s to make design changes. The most basic mechanisms within film projectors, such as the intermittent drive, the shutter, and the projection lens, can be seen in the earliest patents; such as U.S. Pat. No. 93,594 (O. Brown, 1869). Although at present, some manufacturers are producing re-designed projectors with modularity, stepper and servo motor drives, and modern control circuitry, the basic system design is still relatively unchanged. Thus, there continue to be opportunities to make design improvements to a classic opto-mechanical system like the motion picture film projector.
In a standard projector, the film is intermittently advanced by a Geneva Mechanism, also known as a xe2x80x9cMaltese Cross,xe2x80x9d until an image frame is in alignment with the projection aperture. The film is then held stationary for a discrete time period during which light is passed through the aperture, film frame, projection lens, and onto a screen. This intermittent frame-by-frame motion of the film is enabled by the Geneva Mechanism, which comprises one portion, the driver, which rotates continuously, and which causes intermittent rotation of a second portion, the star wheel. In a motion picture projector the star wheel is mounted on a central shaft with a sprocket, the teeth of which are engaged with perforations in the film. Therefore, when the driver moves the star wheel, both the star wheel and the film experience a resulting intermittent motion. As motion picture film is typically projected at a rate of 24 frames per second, a new film frame is positioned in the projection aperture every {fraction (1/24)} second, or approximately 42 ms. The standard Geneva Mechanism used in cinema, much as described is U.S. Pat. No. 1,774,789 (Dina), moves each film frame into the projection aperture with an indexing time of one-fourth of the frame period, or approximately 10.5 ms.
It is necessary to block or shutter the light to the screen during these indexing times to prevent the perception of image smearing or travel ghost by the audience. The typical shutter used in a motion projector is a simple sheet metal disc, which has two blades whose edges extend radially from a center hub, which is mounted to a drive shaft. The shutter is typically positioned between the light source and the film gate, and periodically blocks the light incident to the film through the projection aperture. Shutter design involves a set of trade-off""s around light efficiency, the perception of flicker, and the perception of travel ghost. It happens that human perception of flicker or strobing peaks near the 24 Hz operating frequency of film projectors. To prevent the perception of flicker, the typical shutter has two blades, thereby blocking the light twice per frame (one blockage corresponding to the film indexing time), which raises the apparent illumination frequency to 48 Hz, where flicker perception is significantly reduced. Some systems have even employed three bladed shutters, to yield an effective frequency of 72 Hz, where flicker is barely perceptible. In either of the above cases, these shutters operate at the same 24 Hz rate as the intermittent film driver, and indeed are typically directly linked to the film drive mechanism by a series of worm gears and drive shafts. Alternately, a single bladed shutter driven at 48 Hz or 72 Hz could be employed. Indeed, single bladed shutters are optimal relative to the maximization of screen light and the minimization of travel ghost. However, since single bladed shutters must rotate faster, they generate design, balance, and safety issues, such that they are rarely used. Shutter performance can also be improved by using a shutter disc with a larger diameter, or by positioning the shutter as close as possible to the film plane. In the first case, the shutter blade edges move faster to block the light than is the case for a smaller diameter shutter blade operating at the same speed, and the shutter closure time is reduced. However, size constraints within projector heads typically limit shutters to approx. 4 to 12 inches in diameter. Likewise, physical constraints usually cause the shutter to be positioned an inch or more back from the film gate. Alternately, a shutter with a conical profile has been used as the shutter blade can be positioned closer to the film gate, and the blade velocity across the aperture is more uniform. However, conical shutters have not been widely adopted.
It is a further requirement in shutter design that the multiple blades must be nearly the same size (within a few percent), or else perceptible flicker will be present. Thus, in a cinema projector system employing a standard Geneva mechanism which indexes the film in xc2xc the frame time, the standard two bladed shutter then blocks 50% of the available light from reaching the screen. Thus, rather than make the shutter blades overly large to avoid the appearance of even the slightest amount of image smear, or xe2x80x9ctravel ghost,xe2x80x9d projector manufacturers will use blades which are barely large enough, and then tolerate a small amount of travel ghost.
The perception of travel ghost is a function of both the rate of actual motion of the film as well as the amount of light available to illuminate the film during this motion. It is left to the projectionist to control travel ghost by carefully synchronizing the rotation of the shutter blade with the intermittent action of the Geneva mechanism film driver. For example, travel ghost image smear will appear at the top of the frame when the shutter is late in closing, and will appear at the bottom of the image when the shutter opens too early. Visible travel ghost can occur simultaneously at both the top and bottom of the projected image if both the shutter openings are too large and the shutter is mis-timed with both the beginning and end of the film movement.
A variety of improved shutter designs have been proposed to attempt to maximize light efficiency to the screen while minimizing flicker. For example, the improved shutter described in U.S. Pat. No. 1,700,513 (Porter) has secondary blades, which are mounted to the primary shutter blade disc, and which can be positioned to adjust the size of the shutter openings. By controlling the openings between the radially extending blades in this manner, this shutter is intended to allow both tuning of the light efficiency as well as adjustment for vari-speed projector operation. The shutter described in U.S. Pat. No. 1,884,605 (Dina) also uses a combination of two shutter discs, each with two radially extending blades, whose positions relative to one another can be adjusted to alter the size of the shutter openings, and thus tune available screen light and flicker. In comparison, U.S. Pat. Nos. 3,773,412 and 3,784,293 (Yang) respectively describe shutters with five and four irregularly spaced radially extending blades, where the designed variations in blade position and width are intended to allow maximization of screen light while minimizing flicker.
An alternate approach, described in U.S. Pat. No. 6,014,198 (Baumann) uses a moving plane parallel plate optical compensator, synchronized with the intermittent film movement, to remove the travel ghost effect during shuttering. Accordingly, the screen image of the film appears stationary during a small initial period of time in which the film is actually in motion out of the gate. As during this same initial time period, the shutter blade is already cutting through the illuminating beam, the effective shutter closure time is reduced, although the actual physical closure time is not.
Given these various prior art shutter and projector arrangements, it can be seen advantageous to provide new shutter designs which either reduce the shutter closure time to block light from the projection aperture, or which reduce the time to reach a low light threshold of travel ghost imperceptibility. As a result of reducing either the actual shutter closure time or the travel ghost perception time, the openings of improved shutter can be widened relative to those of the prior art, allowing more light to reach the screen. Furthermore, these improved shutter designs can be combined advantageously with the improved Geneva Mechanism described in the related pending patent application, to further increase screen light. Finally, the improved shutter can be combined with alternate designs for projector illumination systems, to further shorten either the actual shutter closure time or the travel ghost perception time.
It is therefore the object of the present invention to provide shutter blades for a motion picture film projector which reduce shutter closure time and other problems identified above.
It is the further object of the present invention to provide a shutter blade shaped to meet the edge of the projecting aperture, or the aperture edge as projected back into the incident beam, so that the edge is nearly parallel to the aperture edge, or projection thereof.
It is the further object of the present invention to provide a shutter blade shaped to meet the edge of the projecting aperture, or the aperture edge as projected back into the incident beam, while the shutter axis is not located at the traditional position relative to the illumination aperture, which is at 3 o""clock as viewed as the face of a clock, but is shifted relative to the center of shutter rotation.
According to one aspect of the present invention an improved shutter for a motion picture film projector comprises a Geneva Mechanism which intermittently drives a film. A light source projects light through the film and a shutter periodically interrupts the light source. The shutter has a blade with a shaped edge, which matches the frame shape on the film thus, increasing the amount of light that is transmitted through the film. A second edge of the blade also matches the frame shape on the film further increasing the amount of light transmitted through the film.
According to one embodiment of the invention an altered shape, such that the blade edge at the portion of the shutter which cuts through the actual light beam, does not lie along a radial line through the center of curvature of the shutter, but is shaped to meet the edge of the projecting aperture, or the aperture edge as projected back into the incident beam.
Other embodiments include shaping the blade edged to affect the transition times. For example, the leading and trailing edge transition time can be shaped differently. Likewise, the transition can be shaped to not necessarily follow a single slope in time. Also, this blade edge shaping techniques is not limited to two blade shutters, but can be used with one or three blade shutters for example. Also, for example with a two bladed shutter, the two blades may be shaped differently; if one blade is synched with the movement of the intermittent and the film (and is tuned to help with travel ghost), the other blade could be shaped in a different manner, so long as flicker is controlled. Finally, a shaped blade shutter could be constructed with a small blade and servo motor driver instead of as a rotating disc.
An advantage of the present invention is that the combination of blade shaping and aperture/beam shifting results in a faster transition of the shutter as it cuts through the light beam as compared to the standard shutter of the same size.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.